Desalination of seawater by reverse osmosis is a process in which seawater under high pressure, is separated into fresh water and high concentration brine. The separation is achieved by means of membranes that are relatively permeable to water molecules but relatively impermeable to salt ions. Conventional reverse-osmosis desalination units employ one of two techniques for developing this high pressure. Some use high pressure pumps to elevate the feed seawater to between 800 and 1000 psi, while others use centrifuges. Although centrifuges offer theoretically higher efficiencies, practical difficulties have prevented widespread development of centrifugal reverse-osmosis desalination units.
One practical difficulty that has prevented the widespread implementation of centrifugal systems is the design of a membrane configuration suitable for a centrifuge. Siwecki et al., in U.S. Pat. No. 4,333,832 and Wild et al., in U.S. Pat. No. 4,886,597, describe centrifuge designs that accommodate commercially available membrane cartridges. Baram in U.S. Pat. No. 3,840,121; Berriman, in U.S. Pat. No. 3,669,879 and Keefer in U.S. Pat. No. 4,230,564 all describe membrane configurations designed specifically for centrifuge rotors. A principal benefit of this type of membrane configuration is that a greater volume of membrane can be accommodated into a given size of a rotor than by using standard commercially available membrane cartridges. Therefore, the freshwater production from a given size of a rotor is also greater.
Each design described by Baram, Berriman and Keefer are, in some regard, impractical. The membrane configuration proposed by Baram incorporates tubular membranes pressurized internally. Although such membranes are available for low pressure reverse-osmosis applications, tubular membranes that can withstand the high-pressure required for seawater desalination are designed to be pressurized externally only. The membrane configuration proposed by the Keefer patent occupy only a small fraction of the available space within the rotor. The potential for increased productivity from a given size of a rotor is, therefore, not capitalized upon. Various membrane configurations are described by Berriman employ flow patterns which waste rather than recover the energy of the exhaust brine.
One principal benefit of centrifugal reverse-osmosis is that the energy of the exhaust brine is recovered. To achieve this condition, however, the exhaust brine must be returned to the central axis of the rotor before exiting the rotor. In each of Berriman's designs, the feed seawater enters the rotor at the central axis and the concentrated brine exits at the periphery without being returned to the central axis, which is wasteful of the energy in the exhaust brine.
A further problem related to prior art devices is that the reverse osmosis membranes experience concentration polarization which is caused by salt buildup on one side of the reverse osmosis membrane. Many methods have been attempted in the prior art to reduce the negative effects of concentration polarization, each having varying degrees of success.
It is therefore an objective of the invention to provide an apparatus for separating a single solution into at least two solutions, one of which has a lower concentration of a given impurity and one of which has a higher concentration of the impurity than original solution before separation and which utilizes a membrane configuration that is capable of reducing the negative effects of concentration polarization.
In the following discussion the original solution will be termed the "feed solution" and the lower concentration solution will be termed the "product solution," while the higher concentration solution will be termed the "exhaust solution."